Conventional contactors employ mechanical auxiliary contacts according to FIG. 1 to accomplish different kinds of control and monitoring signals. The contact block is constructed with the help of a moving contact element a attached to the contact bridging member, said element carrying a portion of the circuit b of the auxiliary contact, said portion opening and closing the circuit c when said contact element is in either of its home positions. The contacts are sprung to attain longer mechanical life.
A mechanical contact is suitable for controls performed at conventional mains voltage levels, but developments in automation technology have set new demands on the quality of control and monitoring signals. Low voltage level and small current as well as precise timing of the signals are now desirable. Information on the position of a contactor's contact bridging member, for instance, can be signalled by means of mechanical auxiliary contacts, but this prior-art technique involves problems that compromise the reliability of obtained information. Bouncing of contacts at the opening and closing generates signal transients which cause jitter in the precise timing of events, and due to the mechanical play of contacts, the timing of the obtained signal has insufficient accuracy for positional monitoring of the contact bridging member. Moreover, contamination and oxidation of contact surfaces cause malfunctions, particularly at low current and voltage levels.